One type of transducer utilized by the prior art was a flextensional transducer. Flextensional transducers have wider bandwidths, lower operating frequencies and higher power handling capabilities than other types of transducers of comparable size. Flextensional transducers have a single piece, flexible, outer elliptically-shaped shell or housing which is excited by one or more driving elements. The driving elements may be electromagnetic drives, magnetostrictive drives or one or more piezoelectric ceramic stacks. Piezoelectric stacks are driven in a length expander mode and are placed in compression between opposing interior walls of the shell. The elongation and contraction of the piezoelectric stacks imparts a motion to the shell which, in general, radiates or couples energy into the water.
Flextensional transducers are designed to emit sound pressure waves at particular frequencies and power levels. The resonant frequency of the transducer is determined by some characteristics of the shell, namely: the thickness of the shell wall, the length and curvature of the arc of the shell and the ratio between the major and minor axis of the shell. Thus, if the transducer is not resonating at its design frequency, the shape of the shell must be modified.
Single piece shells are expensive and time consuming to manufacture and/or modify because each shell is manufactured and/or modified one at a time by costly manufacturing procedures. The single piece shell was machined from a solid material such as an aluminum or steel alloy or a fiberglass or graphite filament that was fabricated on a mandrel. The above shells could only be modified slightly, since one would be able to thin the wall of the shell, but would not be able to change the length and curvature of the arc of the shell or the ratio between the shell's major and minor axis. It was also difficult to adjust the prestress that was applied to the piezoelectric stacks. The piezoelectric stacks were usually prestressed by placing the transducer shell in a hydraulic press and squeezing the shell across its minor axis while the stacks were placed between the inner major axis walls of the transducer.